const function1 = ()=>{
    return new Promise((resolve,reject)=>{
           resolve('hello ') 
    })
}
const function2 = ()=>{
    return new Promise((resolve,reject)=>{
           resolve('lagou ') 
    })
}

const function3 = ()=>{
    return new Promise((resolve,reject)=>{
           resolve('I love u') 
    })
}
var data=''
function1().then((value)=>{
    data += value
    return function2()
}).then((value)=>{
    data += value
    return function3()
}).then((value)=>{
    data += value
    console.log(data)
})